This invention relates to an apparatus and method for cleaning semiconductor wafers or other such items requiring extremely high levels of cleanliness.
Semiconductor wafers are frequently cleaned in cleaning solution into which megasonic energy is propagated. Megasonic cleaning systems, which operate at a frequency over twenty times higher than ultrasonic, safely and effectively remove particles from materials without the negative side effects associated with ultrasonic cleaning.
Megasonic energy cleaning apparatuses typically comprise a piezoelectric transducer coupled to a transmitter. The transducer is electrically excited such that it vibrates, and the transmitter transmits high frequency energy into liquid in a processing tank. The agitation of the cleaning fluid produced by the megasonic energy loosens particles on the semiconductor wafers. Contaminants are thus vibrated away from the surfaces of the wafer. In one arrangement, fluid enters the wet processing container from the bottom of the tank and overflows the container at the top. Contaminants may thus be removed from the tank through the overflow of the fluid and by quickly dumping the fluid.
A gas impingement and suction cleaning process for electrostatographic reproducing apparatuses which utilizes ultrasonic energy and air under pressure is disclosed in U.S. Pat. No. 4,111,546, issued to Maret.
A process for cleaning by cavitation in liquefied gas is disclosed in U.S. Pat. No. 5,316,591, issued to Chao et al. Undesired material is removed from a substrate by introducing a liquefied gas into a cleaning chamber and exposing the liquefied gas to cavitation-producing means. The shape of the horn to provide the cavitation is not disclosed in detail and does not concentrate the sonic agitation to a particular location within the cleaning vessel.
In U.S. Pat. No. 4,537,511, issued to Frei, an elongated metal tube in a tank of cleaning fluid is energized in the longitudinal wave mode by a transducer that extends through a wall of the tank and is attached to the end of the tube. In order to compensate for relatively high internal losses, the radiating arrangement uses a relatively thin-walled tubular member.
A need exists for an improved apparatus and method which can be used to clean semiconductor wafers.
The above-referenced parent patent application claims various forms of the invention. The present application is directed to additional embodiments of the invention. This includes a housing having an end wall with a vibrator or transducer, such as a piezoelectric transducer coupled to an interior surface of the end wall, while an elongated probe has an end coupled to an exterior surface of the housing end wall. The transducer when energized propagates megasonic energy through the housing end wall and into the probe. The housing is preferably made of aluminum of other material having good thermal conductivity so that heat generated by the transducer is dissipated through the housing.
A liquid coolant passage is formed in heat transfer relation with the housing. Preferably, this is accomplished by positioning a heat transfer member in the housing in a manner to form the passage in combination with an interior wall of the housing. Coolant is then conducted through this passage to provide the desired cooling effect. The transducer is sealed from the liquid coolant, but nitrogen or other gas may be conducted into the housing, preferably through the heat transfer member, to provide gaseous cooling for the transducer.
Preferably, the probe is coupled to the housing by positioning a layer of viscous material between the housing end wall and the probe and pressing the probe against the housing end wall. The pressing force is provided by compressing a spring against the probe with a spring retainer plate which is clamped to the housing. In addition, an O-ring surrounds the interface between the probe and the housing end wall to confine the viscous material and to center the probe, and is clamped in position by a retaining ring mounted to the housing end wall.
A significant advantage of this embodiment is that the housing end wall through which the vibrational energy is transmitted is much thinner than the heat transfer member which is positioned between the probe and the transducer in the other embodiments. This reduced mass allows increased energy to be transmitted to the probe with the same electrical input. The thin wall has fewer internal reflections of energy and makes tuning the transducer easier. Nevertheless, approximately the same cooling capacity is provided with a housing and a heat transfer member having a size comparable to that of the other embodiments.
In another embodiment, a thin wall of a heat transfer member is sandwiched between the transducer and the probe. Surrounding the thin wall is an elongated annular wall of the heat transfer member, which in turn fits snugly within a surrounding cylindrical housing. A channel formed in the exterior surface of the annular wall of the heat transfer member forms a coolant passage in combination with the housing. An O-ring surrounding the base of the probe is held in position by a retaining ring fastened to the end of the housing. Since the base or rear of the probe extends into the housing, the retainer ring includes a cylindrical portion that extends into the housing to engage the O-ring. An advantage of this embodiment of the invention is that the megasonic energy can be efficiently transmitted by the transducer through the thin wall of the heat transfer member into the probe, while at the same time providing excellent cooling characteristics because of a direct path from the transducer through the heat transfer member to the cooling fluid passage.
Another aspect of the invention that applies to all the embodiments discussed above, as well as those in the parent application, is a probe made of a base having a socket formed therein for receiving an elongated rod portion preferably having a constant cross-section. The base and rod are coupled in a manner to efficiently transmit the megasonic energy from the base to the rod. An advantage of this arrangement is that a single base can be used for rods of different materials and lengths which are to be contacted by a cleaning fluid. For example, a quartz base may be used for many applications, while a rod of different material may be used with a solution containing hydrofluoric acid, which is not compatible with quartz. Also, some materials such as vitreous carbon can withstand most cleaning solutions but the material is presently only readily available in constant diameter rod form.